It has been proposed to manufacture an accelerometer in one process step within the plane of a silicon wafer by a deep trench etching process. Such a proposed accelerometer is shown in plan view in FIG. 1 of the accompanying drawings. This proposed accelerometer employs a substantially planar, ring-like support frame 1 made from silicon which is anodically bonded to a plate-like base 2 as shown in FIG. 2 of the accompanying drawings. FIG. 2 is a cross-sectional view taken on the line A-A in FIG. 1.
The accelerometer of FIGS. 1 and 2 has a substantially planar plate-like proof mass 3 made from silicon which is moveably mounted in the support frame 1 and co-planar therewith whilst being separated from the underlying glass base 2. The proof mass 3 is carried on four or more flexible mounting legs 4 made from silicon each co-planar with the proof mass 3 and the support frame 1. Each mounting leg 4 is connected at one end 4a to the proof mass and connected at another end 4b to the support frame 1 so that the proof mass 3 is mounted for linear movement in a sensing direction B in the plane containing the support frame 1, proof mass 3 and mounting legs 4, in response to acceleration change applied to the accelerometer. The mounting legs 4 extend substantially perpendicularly to the sensing direction B.
A plurality of interdigitated capacitor fingers 9 shown simplistically in FIG. 1 are mounted in a gaseous medium in the support frame 1 for sensing linear movement of and for providing gaseous medium squeeze damping for, the proof mass 3 in the sensing direction B, with the fingers 9, proof mass 3, mounting legs 4 and support frame 1 being co-planar and formed from a single plate of mono-crystalline silicon. The fingers 9 comprise fixed first, second, third and fourth arrays 5, 6, 7 and 8 respectively of laterally spaced fingers 9 extending substantially perpendicularly to the sensing direction B and away from the support frame 1 towards the proof mass 3. The first and second arrays 5, 6 are located on one side of the proof mass 3 and the third and fourth arrays 7, 8 are located on the opposite side of the proof mass 3 as shown in FIG. 1. The fingers of the first, second, third and fourth arrays 5, 6, 7 and 8 are fixedly bonded to the base 2. Also provided are moveable fifth, sixth, seventh and eighth arrays, 10, 11, 12 and 13 of laterally spaced fingers 9 extending substantially perpendicularly to the sensing direction B from and attached to the proof mass 3 towards to the support frame 1 with the fifth and sixth arrays 10, 11 being located on said one side of the proof mass 3 and interdigitated respectively with the first and second arrays 5, 6 and with the seventh and eighth arrays 12, 13 being located on said opposite side of the proof mass 3 and interdigitated respectively with the third and fourth arrays 7, 8. The interdigitation of the first and fifth arrays 5, 10 and of the third and seventh arrays 7, 12 is at a first offset 14 in one direction in the sensing direction B from a median line between adjacent fingers in the first, second, third and fourth array 5, 6, 7 and 8 and the interdigitation of the second and sixth arrays 6, 11 and of the fourth and eighth arrays 8, 13 is at a second offset 15 in the opposite direction to the first offset 14.
The fingers 9 have a narrow gap with atmospheric pressure gas between them, typically air. Under deflection of the proof mass 3 the gas can move and the viscosity of the air sets the rate at which the gas moves and thus the damping force. Additionally the interdigitated fingers 9 of the eight arrays form two sets of sensing capacitors between the fixed support frame 1 and the proof mass 3. Typically the first array 5 and third array 7 of fingers form a first or upper terminal and the second array 6 and fourth array 8 form a second or base terminal for the capacitors. The fixed fingers 9 of the arrays 5, 7, 6 and 8 are electrically isolated from the proof mass 3 and anodically bonded to the base 2 so that the interdigitated fingers form the sensing and forcing capacitors. The sensing capacitors fingers are offset so that when a voltage is applied between the first terminal, and the proof mass 3 or between the second terminal and the proof mass 3, there is a net attractive force depending on the sign of the voltage difference. Without an offset there would be no net force.
The support frame 1 has a substantially rectangular ring-like shape surrounding an inner open area in which is located the proof mass 3 which has a substantially rectangular shape. The mounting legs 4 extend substantially perpendicularly to the sensing direction in spaced array with at least two extending between an inner wall of the support frame 1 defining the inner open area and a facing outer wall of the proof mass 3 and with at least two extending between an opposing second inner wall of the support frame 1 defining the open inner area and a facing second outer wall of the proof mass. At least four earth screens 16 are provided located within the inner open area and each being associated with and partially surrounding a respective one of the first, second, third and fourth arrays, 5, 6, 7 and 8 of fingers and being operable to shield the arrays of fingers from the support frame 1 and being electrically insulated from the support frame 1. As the fixed capacitor fingers have a high alternating current voltage applied, for synchronous demodulation, it is this signal frequency that appears on the proof mass 3 determining the positional offset. It is thus necessary to screen the high AC voltage drive from the small AC signals on the proof mass. The earth screens are held at zero voltage so as to minimise the cross-coupling from the drive to the sensing signals. The earth screens 16 are anodically bonded through to the base 2 for good electrical isolation.
One of the key problems of this proposed accelerometer is the differential expansion rate between the silicon from which the support frame 1, proof mass 3, mounting legs 4 and fingers 9 are composed and the material of the base 2, preferably glass. This causes flexure of the accelerometer assembly that alters the tensile force along the mounting legs 4. Any longitudinal force on the silicon will apply an additional tensile force to the legs 4 causing the resonance frequency to go up and the scale factor to go down and vice versa. Thus due to the differential expansion between the silicon and the underlying base 2 as the temperature is changed a bimetallic effect is set up which gives rise to a concave or convex bow of the accelerometer assembly. Compressive and tensile forces on the mounting legs 4 are very undesirable as they cause a degradation of accuracy of the accelerometer and a reduction of operational stability over an operating temperature range. Additionally the scale factor of the accelerometer is changed with the changes in tension and compression.
There is thus a need for a generally improved accelerometer which at least minimises the foregoing disadvantages inherent in the proposed accelerometer.